Uniformitarianism - “The present is the key to the past.” Original Horizonality - “Sedimentary layers are deposited in a horizontal or nearly horizontal position.” Superposition - “Younger sedimentary beds occur on top of older beds, unless they have been overturned or faulted.” Cross-Cutting Relations - “Any geologic feature that cuts another geologic feature is younger than the feature that it cuts.”

Igneous Rocks
Comprise 95% of the Earth's crust. Originated from the solidification of molten material from deep inside the Earth. There are two types: •Volcanic - glassy in texture due to fast cooling. •Plutonic - slow-cooling, crystalline rocks.

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Igneous Rocks and Reservoirspart of reservoirs. Igneous rocks can be
Fractured granites form reservoirs in some parts of the world. Volcanic tuffs are mixed with sand in some reservoirs.

Example: Granite Wash - Elk City, Okla., Northern Alberta,CA

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Metamorphic Rocks
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2) Metamorphic rocks formed by the action of temperature and/or pressure on sedimentary or igneous rocks. Examples are
• • •

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Marble - formed from limestone Hornfels - from shale or tuff Gneiss - similar to granite but formed by metamorphosis

These are the most important for the oil industry as it contains most of the source rocks and cap rocks and a majority of the reservoirs. Sedimentary rocks come from the debris of older rocks and are split into two categories Clastic and Non-clastic.
•

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Clastic rocks - formed from the materials of older rocks by the actions of erosion, transportation and deposition. Non-clastic rocks - from chemical or biological origin and then deposition.

The depositional environment can be Shallow or deep water. Marine (sea) and lake or continental. This environment determines many of the reservoir characteristics

Frigg Gas Field - North Sea
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Depositional Environments
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Continental deposits are usually dunes. A shallow marine environment has a lot of turbulence hence varied grain sizes. It can also have carbonate and evaporite formation. A deep marine environment produces fine sediments.

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Depositional Environments
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The depositional characteristics of the rocks lead to some of their properties and the reservoir property.
• •

The reservoir rock type clastic or non-clastic. The type of porosity (especially in carbonates) is determined by the environment plus subsequent events.

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The structure of a reservoir can also be determined by deposition; a river, a delta, a reef etc. This can also lead to permeability and producibility of these properties are often changed by further events.

Sediments are transported to the basins by rivers. A common depositional environment is the delta where the river empties into the sea. A good example of this is the Mississippi (Miocene and Oligocene sands)

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Rivers

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Some types of deposition occur in rivers and sand bars. The river forms a channel where sands are deposited in layers. Rivers carry sediment down from the mountains which is then deposited in the river bed and on the flood plains at either side. Changes in the environment can cause these sands to be overlain with a shale, trapping the reservoir rock.
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Carbonates (limestone and dolomite) normally have a very irregular structure. Porosity:
•

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Determined by the type of shells, etc. and by depositional and post-depositional events (fracturing, leaching, etc.).

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Permeability:
•

Determined by deposition and post-deposition events, fractures.

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Fractures can be very important in carbonate reservoirs.

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Carbonate types
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Chalk is a special form of limestone (CaCO3) and is formed from the skeletons of small creatures (cocoliths). Dolomite (CaMg(CO3)2) is formed by the replacement of some of the calcium by a lesser volume of magnesium in limestone by magnesium. Magnesium is smaller than calcium, hence the matrix becomes smaller and more porosity is created. Evaporites such as Salt (NaCl) and Anhydrite (CaSO4) can also form in these environments.

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Depositional Environment Carbonates

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Carbonates are formed in shallow seas containing features such as:
• • •

Reefs. Lagoons. Shore-bars.

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Diagenesis
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The environment can also involve subsequent alterations of the rock such as:
• •

Chemical changes. Diagenesis is the chemical alteration of a rock after burial. An example is the replacement of some of the calcium atoms in limestone by magnesium to form dolomite.

•

Mechanical changes - fracturing in a tectonically-active region.

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Hydrocarbon Generation, Migration, and Accumulation

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Source Rocks
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Hydrocarbon originates from minute organisms in seas and lakes. When they die, they sink to the bottom where they form organic-rich "muds" in fine sediments. These "muds" are in a reducing environment or "kitchen", which strips oxygen from the sediments leaving hydrogen and carbon. The sediments are compacted to form organic-rich rocks with very low permeability. The hydrocarbon can migrate very slowly to nearby porous rocks, displacing the original formation water.

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Hydrocarbon Migration

Hydrocarbon migration takes place in two stages: Primary migration - from the source rock to a porous rock. This is a complex process and not fully understood. It is probably limited to a few hundred metres. Secondary migration - along the porous rock to the trap. This occurs by buoyancy, capillary pressure and hydrodynamics through a continuous water-filled pore system. It can take place over large distances.

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Vitrinite
A nonfluorescent type of organic material in petroleum source rocks derived primarily from woody material. The reflectivity of vitrinite is one of the best indicators of coal rank and thermal maturity of petroleum source rock.

Faults occur when the rock shears due to stresses. Reservoirs often form in these fault zones. A porous and permeable layer may trap fluids due to its location alongside an impermeable fault or its juxtaposition alongside an impermeable bed. Faults are found in conjunction with other structures such as anticlines, domes and salt domes.

The “4” in 4-D seismic is time A 4-D survey means that at least two 3-D seismic surveys have been made at different times over the same field Reflection character (attributes) change through time These changes result from migration of the water contact in the reservoir

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Exercises: Petroleum Geology

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Exercise 1
1. 2. 3. 4. 5. 6. Oil forms at lower temperatures than gas. T_____ F ______ The law of (original horizontality, uniformitarianism, superposition) states that, in a normal sedimentary sequence, younger layers occur on top of older layers. The largest division of geologic time is the (era, eon, period, epoch). Hydrocarbons are most abundant in (metamorphic, igneous, sedimentary) rocks. The most abundant sedimentary rock type is shale. T____ F ______ Name 3 clay minerals common in sandstone reservoirs A. _____________________ B.____________________
7.

C. ____________________

Clastic rocks are formed from the materials of older rocks by the actions of erosion, transportation and __________________. Clastic rocks are sedimentary. T___ F____ Name two non-clastic sedimentary rocks. A.______________ B.________________ Alluvial, desert, delta, beach and shallow shelf sediment make the best reservoirs T_______ F_______

8.

9.

10.

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Exercise 2
1. 2. 3. 4. 5. 6. 7. 8. 9. 1. Diagenesis is the chemical alteration of a rock after burial. T___ F ___ (Magnesium, Iron, or Sulfate) must be in the formation water in order to convert limestone to dolomite. Limestone is (CaCO3 or Ca(CO3)2). Dolomite is MgCaCO3 or MgCa(CO3)2. Reef deposits are classified as (clastic, carbonate) sedimentary rocks. The source rock must contain (organic material, coal, methane). Fault and anticline traps occur only in gas wells. T___ F___ The oil water contact can be observed using seismic T___ F___ (Historical, structural, tectonic) geology addresses the occurrence and origin of smaller scale deformational features, such as folds and faults, that may be involved in hydrocarbon migration or which may form structural hydrocarbon traps. Good quality sandstone reservoirs normally contain ~ (1-10 or 25-30% silt and clay).